NAME:______

From Gene to Protein – Transcription and Translation[i]

In this activity you will learn how the genes in our DNA influence our characteristics. For example, how can a gene cause albinism (very pale skin and hair)?

Basically, a gene is a segment of DNA that provides the instructions for making a proteinand proteins influence our characteristics. This chart describes how two different versions of a gene can result in either normal skin and hair color oralbinism.

DNA /  / Protein /  / Characteristic
/  / /  /
Version of the gene that provides instructions to make
normal protein enzyme /  / Normal enzyme that makes the pigment molecule in skin and hair /  / Normal skin and hair color
Version of the gene that provides instructions to make
defective enzyme /  / Defective enzyme that does not make this pigment molecule /  / Albinism (very pale skin and hair)

A gene directs the synthesis of a protein by a two-step process.

The first step is transcription of the gene in the DNA. Transcription produces a messenger RNA (mRNA)molecule.
The second step is translation of the mRNA.
Translation produces a protein molecule. /
During transcription, the sequence of nucleotides in the gene in the DNA is copied into a corresponding sequence of nucleotides in mRNA.
During translation, the sequence of nucleotides in the mRNA determines the sequence of amino acids in the protein. /

After translation, the sequence of amino acids in the protein determines the structure and function of the protein.

Notice thatDNA and RNA are polymersof four types of nucleotides, A, C,G,and Tfor DNAand A, C, G,and U for RNA. In contrast, proteins are polymers of 20types of amino acids.

1.To summarize how a gene directs the synthesis of a protein, label the process indicated by each arrow and fill in the blank with the appropriate molecule.

Gene in DNA ______protein

2. Complete the following sentence to describe how differences in a gene can result in normal skin and hair color vs. albinism.

Differences in the sequence of ______in the gene

result in differences in the sequence of ______in mRNA which

result in differences in the sequence of ______in the protein which

result in normal vs. defective enzyme to make the pigment in skin and hair which

results in normal skin and hair colorvs.______.

Transcription

During transcription,each DNA nucleotide in the gene is matched with a complementaryRNAnucleotide following the complimentary base-pairing rules. The base-pairing rules for transcriptionare very similar to the base-pairing rules in the DNA double helix.

Use the information in the figure to complete the following table. Remember, the bases of DNA nucleotides are A, T, G and C. The bases of RNA nucleotides are A, U, G, and C.

Base-Pairing Rules for Complementary Nucleotides:
between two strands of DNA / between DNA and RNA
(during transcription)
G pairs with______. / G pairs with ____.
T pairs with ______. / T in DNA pairs with ____ in RNA.
A in DNA pairs with ____ in RNA.

The base-pairing rules ensure that the message from the nucleotide sequence in thegene in the DNA is copied into a corresponding nucleotide sequence in the mRNA molecule.

Fill in the blanks in this table to summarize the differences between DNA replication and transcription.

DNA replication / Transcription
The whole chromosome is replicated. / The information for one ______is transcribed.
DNA is made.
DNA is ______-stranded. / mRNA is made.
mRNA is ______-stranded.
T = thymine is used in DNA,
so ______pairs with T in DNA. / T = thymine is replaced by ___= uracil in RNA,
so A in DNA pairs with ___ in mRNA.

Translation

As you know, transcription is followed by translation. During translation, the sequence of nucleotides in mRNA determines the sequence of amino acids in a protein.
Each set of three nucleotides in an mRNA molecule codes for one amino acid in a protein. This explains why each set of three nucleotides in the mRNA is called a codon. Each codon specifies a particular /

amino acid.

Fill in the blanks to complete the following sentence.

In the figure above, the first codon, _____, codes for the amino acid ____ (arginine) in the protein.

But how is translation accomplished in a cell? In Translation, a special type of RNA, transfer RNA (tRNA), is required to ensure that the correct amino acid is brought in for each codon in the mRNA.

There are multiple different types of tRNA. Each type of tRNA molecule has three nucleotides that form an anti-codon. The three nucleotides in the tRNAanti-codonare complementaryto the three nucleotides in the mRNA codon for a specific amino acid. For each type of tRNA, there is a specific enzyme that recognizes the anti-codon and attaches the correct amino acid to the tRNA .

A ribosome is a tiny organelle where protein molecules are synthesized. The tRNA with amino acid enters the ribosome where the anti-codon in the tRNA is matched with a codon in the mRNA molecule. The tRNA brings the correct amino acid for that position in the growing protein molecule. Each amino acid is joined to the previous amino acid by a peptide bond. The ribosome moves along the mRNA, matching each codon with a complementary tRNA anti-codon and adding the appropriate amino acids one at a time to produce the protein coded for by the mRNA.

Circle the anti-codon in one tRNA molecule in the figure. In the ribosome, put a rectangle around an anti-codon in a tRNA and the complementary codon in the mRNA.

Amino acid / Threonine (Thr) / Histidine (His) / Proline (Pro) / Leucine (Leu) / Glutamic acid (Glu) / Valine (Val)
mRNA codon / ACU / CAU / CCU / CUG / GAG / GUG
Anti-codon in tRNA molecule that carries this amino acid / UGA

Modeling Protein Synthesis

During transcription, one side of the DNA molecule is used to make a mRNA molecule. We call this side of the DNA the template strand. Below you will use the rules of complimentary base paring to determine what mRNA molecule will be made, and in turn, what protein will be made. Each set of 3 bases on the mRNA is called a codon. The codon is specific for a particular amino acid. You can use charts like the one below to determine what amino acids go with what codons.The codons on the mRNA match by complimentary base paring with the bases on the tRNA molecules. We call the three complimentary bases on the tRNA the anticodon.

Directions:

  1. In line 1 write the complimentary DNA base pairs that would complete the DNA molecule.
  2. In line 2 write the sequence of bases of the mRNA molecule that would be made during transcription. Assume that the bottom row of DNA nucleotides is the template strand.
  3. In line 3 determine what bases would need to be on the tRNA molecules in order to bring the correct amino acids to the ribosome.
  4. In line 4, use the codon wheel above to determine which amino acids will be linked together to make the protein.

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How the Gene for Sickle Cell Hemoglobin Results in Sickle Cell Anemia

Different versions of the same gene are called different alleles. These different alleles share the same general sequence of nucleotides, but they differ in at least one nucleotide in the sequence.

Different alleles can result in different characteristics as follows:

Different alleles of a gene have a different nucleotide sequence

differentnucleotide sequence in messenger RNA (mRNA)

transcription

different amino acid sequence in a protein

translation

different structure and function of the protein

(e.g. normal enzyme vs. defective enzyme)

different characteristics

(e.g. normal skin and hair color vs. albinism)

In this section, you will learn about another example of how different alleles produce different characteristics.To begin, you will work to understand how a difference between the alleles for normal and sickle cell hemoglobinresults in two different types of hemoglobin protein.

21.In the table below, compare the nucleotide sequence in the DNA for the Beginning of the Normal Hemoglobin Gene vs. the Beginning of the Sickle Cell Hemoglobin Gene. What is the only difference?

22.Complete this table. (Use thetable on page 6 to help with translation.)

Beginning of Normal Hemoglobin Gene / CACGTAGACTGAGGACTC
Transcription produces: / codon1 / codon2 / codon 3 / codon 4 / codon 5 / codon 6
Beginning of Normal Hemoglobin mRNA
Translation produces: / amino acid 1 / amino acid 2 / amino acid 3 / amino acid 4 / amino acid 5 / amino acid 6
Beginning of Normal Hemoglobin Protein
Beginning of Sickle Cell Hemoglobin Gene / CACGTAGACTGAGGACAC
Transcription produces: / codon 1 / codon 2 / codon 3 / codon 4 / codon 5 / codon 6
Beginning of Sickle Cell Hemoglobin mRNA
Translation produces: / amino acid 1 / amino acid 2 / amino acid 3 / amino acid 4 / amino acid 5 / amino acid 6
Beginning of Sickle Cell Hemoglobin Protein

23.Compare the beginning of the hemoglobin molecules synthesized by translating the sickle cell vs. normal hemoglobin mRNA molecules. What difference in the amino acid sequence do you observe?

Sickle cell hemoglobin and normal hemoglobin differ in only a single amino acid out of more than 100 amino acids in the complete hemoglobin protein. This difference in a single amino acid results in the very different properties of sickle cell hemoglobin, compared to normal hemoglobin.

If a person inherits two copies of the sickle cell hemoglobin allele and produces only sickle cell hemoglobin, then the sickle cell hemoglobin molecules tend to clump together in long rods. These rods can change the shape of the red blood cells from their normal disk shape to a sickle shape. Sickle-shaped red blood cells can block blood flow in the smaller blood vessels. This causes pain and damage to body organs. In addition, sickle-shaped red blood cells do not last nearly as long as normal red blood cells, so the body cannot produce enough replacement red blood cells and the person develops anemia (not enough red blood cells).

Genotype (genes) /  / Protein /  / Phenotype (characteristics)
2 copies of the allele
that codes for
normal hemoglobin
(SS) /  / Normal hemoglobin dissolves in the cytosol of red blood cells.
/  / Disk-shaped red blood cells can squeeze through the small blood vesselsnormal health

2 copies of the allele
that codes for
sickle cell hemoglobin (ss) /  / Sickle cell hemoglobin
can clump in long rods
in red blood cells.
/  / When sickle cell hemoglobin clumps in long rods
sickle-shaped red blood cells
clogged small blood vessels
+fragile red blood cells
pain, damage to body organs
+ anemia = sickle cell anemia

24.Circle the arrows in the chart that represent transcription + translation.

In summary, the sickle cell allele results in production of the sickle cell hemoglobin protein, which results in the health problems observed in sickle cell anemia. This illustrates the importance of the nucleotide sequence in a gene, which determines the amino acid sequence in a protein, which in turn influences the characteristics of an individual.

25. Considering that we are all made up of the same 4 nucleotides in our DNA, the same 4 nucleotides in our RNA, and the same 20 amino acids in our proteins, why are we so different from each other? For example, why do some people have sickle cell anemia and others don't?

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